1. Field of the Invention
The invention relates to a method of printing an image, built up from pixels, on a substrate, comprising moving a print head having at least two image-forming elements with respect to the substrate and activating said elements image-wise, in order to provide the pixels with image-forming material, at least one element having broken down so that at least one pixel is not provided with the image-forming material. The invention also relates to a printing device adapted to performing this method.
2. Description of the Related Art
A method of this kind is known from the Laid-Open Japanese Patent Application 60104335. In this method, use is made of an ink jet printer provided with a print head having a number of main image-forming elements, each element comprising the outflow opening of an ink duct, and reserve image-forming elements. A substrate is printed by providing pixels of a required image with ink drops generated by the main image-forming elements. If during printing an xe2x80x9cabnormality detectorxe2x80x9d shows that there has been a breakdown of a main image-forming element, a reserve element is put into use instead of the broken-down main element in order that the associated pixels may nevertheless be provided with an ink drop. This prevents the need to interrupt the printing process in order to repair the broken-down element. A significant disadvantage of this method is that the print strategy has to be adapted in order that the associated pixels which could not be printed may nevertheless be provided with an ink drop via a reserve element, and this is at the expense of the productivity of the printing device. Another disadvantage is that the printing device must be provided with a number of reserve image-forming elements in addition to the main image-forming elements.
Another method of this kind is also known from US Pat. No. 4,963,882. This method, which makes use of an ink jet printer, proposes reducing the visible effect of the breakdown of an image-forming element. To this end, ink drops originating from different image-forming elements are printed on one pixel or one pixel row. In the former case, in which ink drops originating from different elements are printed on one pixel, a pixel is provided with two ink drops as a standard, this procedure being known as DOD xe2x80x9cdot-on-dotxe2x80x9d or DDA xe2x80x9cdouble-dot-alwaysxe2x80x9d. In the event of breakdown of one of the associated elements, the pixel is nevertheless always provided with one of two ink drops. In this way the visible effect of the failure of the associated image-forming element is practically nil. This method has the significant disadvantages that the productivity of the printing device is only half the maximum productivity, since each pixel must be provided with two ink drops, and also more ink is consumed per unit area of a substrate.
In the second case, in which ink drops originating from different elements are printed on one pixel row, each pixel is provided with only one ink drop, but two elements are used for each pixel row, so that different pixels in one row are provided with ink drops originating from two different elements. Frequently, each pixel row is so filled that pixels are provided one by one with ink drops originating from one or other element. In the event of failure of one of the two elements, it will be possible to provide a pixel row with the ink drops originating from the other element, so that not all the information of the pixel row needs to be lost. A significant disadvantage of this method is that on average 50% of the information in a pixel row is lost in the event of breakdown of one of the two image-forming elements. Depending on the image that should have been formed, as much as 100% of the information of the associated pixel row may be lost.
The object of the invention is to obviate these disadvantages. To this end, a method has been invented wherein by activating an element other than the said broken-down element a correction point in the vicinity of the said at least one pixel is provided with the image-forming material, the correction point not coinciding with a pixel. In other words, information which would be lost because of breakdown of an image-forming element, without the original print strategy being adapted, is transferred to a nearby addressable correction point.
This method has a number of important advantages. Firstly, the print strategy does not have to be adapted, so that using this method does not cost any productivity. In the method according to the invention, a correction point is printed in one of the original printing steps and not in an extra printing step in which extra step the associated pixel is being provided with image-forming material from another element. In addition, no reserve image-forming elements are required. Moreover, there is no need to provide each pixel with two or more drops of image-forming material so that the printing device again does not lose any productivity as a result. Finally, no information has to be lost if the method according to the invention is applied.
Although the information is printed at a location differing from an originally intended pixel, it is invisible or practically invisible to the human eye since it takes place in the vicinity of the associated pixel. The image formed on a substrate will therefore be substantially identical to the originally intended image. In a preferred embodiment, the correction point adjoins the pixel that cannot be printed by the broken-down image-forming element. This means that the correction point is selected from the group of printable locations which together surround the pixel. In this way the visible consequences of a broken-down ink duct are practically nil.
The invention also relates to a printing device adapted to performing the method according to the invention. In a preferred embodiment the printing device is an ink jet printer.
If the printing device can print a plurality of colours, for example cyan, magenta, yellow and black, in order to form a total image overall from a number of images each consisting separately of one of said colours, the method is applied for each colour image separately. In the event, for example, of breakdown of an image-forming element with which the colour cyan is printed, then for a pixel which as a result cannot be provided with a quantity of image-forming material in the colour cyan originating from the said broken-down image-forming element, a correction point will be selected which does not coincide with a cyan pixel. This correction point may well coincide with a pixel of a different colour. The choice for this is determined, among other reasons, by the distribution of the colour images over the substrate. For example, in certain cases it may be favourable to select a correction point which also does not form part of any other colour image while in other cases it may be favourable to select a correction point that in fact is a part of one of the other colour images.
No general strategy can be given for selection of a suitable correction point, i.e. a correction point such that the visible consequences of the breakdown of an image-forming element are minimal. In addition to depending on the colour composition of the required image as indicated hereinbefore, the strategy is dependent inter alia on the selected print strategy, the geometry of the print heads of the printing device, the from of image, the font size, the area coverage, the image processing method, the type of half-toning and so on. In the examples hereinafter, the invention will be explained in detail with reference to an ink jet printer adapted to application of the method according to the invention, a number of these points being discussed in greater detail.